Seed supply
Farmers obtain seed from their own parks (protected bottoms) or from the natural clam populations in the spring. They dig the clam seed with sand using a small shovel, pass it through a sieve to retain the seed, take it to their ongrowing parks, and spread it in densities of about 800 clams/m
2. They may also dig adult clams from seaport areas and spread them in their parks. Periodically, they have to clean their parks of predators and mud.
Hatchery production
Seed can also be obtained from hatcheries, where breeders, not exceeding 40 mm are maintained for 30-40 days at 20 °C. Breeders are fed with unicellular algae until the induction of the liberation of gametes. Gamete liberation can be induced by raising the temperature from 10 to 26 °C, maintaining it at that temperature for 30 minutes and then reducing it to 15 °C for several minutes; the cycle of raising temperature to 26 °C and lowering it again is then repeated until gametes are liberated. The addition of sperm from a sacrificed animal may also help in liberation. Fertilization occurs in small containers where the animals are isolated from each other. Eggs are filtered through a 40 µm mesh, and transferred to a 10 litre tank, where veliger larvae appear after 48 hours. Larvae are recollected in a 40 µm mesh and reared at densities of 3 000 larvae/litre. They are fed with unicellular algae every day during the first week and then every second day until metamorphosis.
Nursery
Clams can be reared in nurseries within greenhouses, with controlled feeding by using unicellular algae, or reared in meshed containers over culture tables. An alternative is to pump environmentally controlled water to inland tanks where clams are placed in cylindrical containers of about 50 cm diameter and 20 cm long, with a bottom made of a rigid mesh.
Ongrowing techniques
Culture techniques are simple, consisting of seeding juvenile clams; regular maintenance of the substrate; avoiding seaweed and predators such as crabs and starfish; oxygenating the substrate with shovels and rakes and discarding seaweed growth; and maintaining an appropriate clam density. Ongrowing requires the clams to be submerged; burrowing reduces the level of shell malformations. Most clams become submerged in the substrate within an hour after sowing. Clams can either be placed in beds without any control other than periodic cleaning or in structures such as nets that may minimize predator impact but increase maintenance costs. In summary, the culture technique is simple.
Harvesting techniques
In the Galician region (Spain), fishermen harvest clams by walking the intertidal areas and using special hand shovels, or sometimes by using the rakes that are normally used for keeping the culture beds clear of seaweed. Clams may also be harvested from boats, which may vary in size between less than 1 tonne and up to 12 tonnes. Some are propelled with oars, others with outboard engines. Various collection tools are used, including the 'rastro' and the 'raño' (rake), which are operated from the boats with the help of a long handle. The closed season is from March to October, and the minimum size allowed for
Venerupis pullastra is 25 mm. Some Galician areas have protected bottoms called 'parks' for the extensive culture of clams. In hand (walking) harvesting clams are harvested with the help of different types of small shovels; sometimes the rakes that are usually more employed for cleaning the parks of seaweeds are employed.
Handling and processing
Fishermen bring their clams to depuration stations where they are held in tanks for at least 42 hours. The clams are then packed in net bags of 0.5 - 1 and 2 kg, and are destined to be canned or eaten fresh. They are transported by refrigerated trucks which maintain their temperature at 3-10 °C; the clams have a shelf life of 5 days.
Production costs
Production costs are greatly influenced by the socio-economic environment and the size of seed supplied. If nursery time starts in spring, harvesting occurs in the late fall or early winter of the following year. Important factors in total costs are seabed leases or exploitation charges hatchery and nursery facility costs, management and harvesting tools and labour.
Perkinsosis; Clam Perkinsus disease |
Perkinsus olseni; P. atlanticus |
Protistan parasite |
Visible milky white cysts or nodules on the gills, foot, gut, digestive gland, kidney, gonad & mantle of heavily infected clams; may induce massive mortality |
Clams from areas with records of the disease should not be transplanted; reduce density |
Icosahedrical virus-like disease of carpet shell clams |
Icosahedrical virus-like organism |
Virus |
Impact on host not specifically described but mortality can be significant |
None known |
Brown ring disease |
Vibrio tapetis |
Bacterium |
Bacteria adhere to surface of periostracal lamina at mantle edge of shell & progressively colonize the resulting secretion causing brown deposit of organic material (conchiolin) adhering to inner surface of shell; normal calcification process disturbed; growth stunted |
Clams from areas with records of the disease should not be transplanted; reduce density |
Ciliate infestation |
Trichodina sp. |
Trichodinid ciliate |
Most infections are innocuous; intensity of infection usually low with ciliates attached to, or located near mantle, along siphons, or gill surface |
No known methods of prevention or control |
Turbellarian infestation |
Paravortex sp. |
Flatworm parasite |
No known affect on host; thought to pass freely between mantle cavity and alimentary canal; often observed on gill surface; rarely observed in lumen of kidney; probably midway between being endocommensal and parasitic |
Prevention and control impractical |
Trematode infestations |
Unidentified cercarias and metacercarias |
Trematodes |
Generally innocuous |
Prevention and control impractical |
Among bivalve competitor species growing with
Venerupis pullastra are
Venerupis rhomboideus,
Venerupis aurea,
Ruditapes decussatus,
Dosinia exoleta and
Tellina incarnata. Their main predators are the shore crab (
Carcinus maenas); starfishes (
Asteria rubens and
Marthasterias glaciais); gastropods (
Natica sp.); and birds (
Larus sp). An individual shore crab (6.5 cm width) can consume 5-6 clams per day. Nets are used in some culture areas to control predators.
Suppliers of pathology expertise
Expertise in pathology can be obtained from:
Dr. Antonio Figueras
Instituto Investigaciones Marinas, CSIC Spanish National Reference Laboratory for Mollusc Diseases
Eduardo Cabello 6 - 36208 VIGO - Spain
Telephone: +34 986 21 44 62 -
Fax: +34 986 292 762
E-mail:
[email protected]
Dr. Isabelle Arzul
IFREMER, Laboratoire de Génétique et Pathologie
BP 133 - 17390 La Tremblade - France
Telephone: +33 5 46 36 98 43 -
Fax: +33 5 46 36 37 51
E-mail:
[email protected]
Dr. E.M. Burreson
Director for Research and Advisory Services Virginia Institute of Marine Science College of William and Mary
P.O. Box 1346 Gloucester Point - VA 23062 - United States of America
Telephone: +1 804 684 7015 -
Fax: +1 804 684 7097
E-mail:
[email protected]
Dr. S. Bower
Department of Fisheries and Oceans Pacific Biological Station3190 Hammond Bay Road Nanaimo
British Columbia V9R 5K6 -
Canada
Telephone: +1 250 756 7077 -
Fax: +1 250 756 7053
E-mail:
[email protected]
Dr. Jorge Cáceras Martínez
Instituto de Sanidad Acuícola, A.C.
Calle 9 y Gastelum # 468,13,14. CP. 22800 -
Ensenada Baja California - Mexico
Telephone: +52 646 1783473 -
Fax: +52 646 1783473
E-mail:
[email protected]